All of Jean Grey's powers derive from her raw psychic ability (telekinesis, telepathy etc) and there's reasonable evidence in the X-Men Cinematic Universe that her raw power level in this regard completely outstrips everybody (including Xavier) when it is allowed to operate unrestrained.
As we see in X-Men: The Last Stand she can wield telekinetic forces strong enough to disintegrate people, tanks etc. with very little difficulty, reach out telepathically across vast distances without the aid of Cerebro (such as when she "calls" to Cyclops from Alkali Lake)
Her attack on Apocalypse is primarily a psychic one - she enters Xavier's mind (where he is mentally fighting Apocalypse) in order to help and starts attacking him psychicly herself, it is this psychic onslaught that distracts/weakens Apocalypse enough to allow the others (Storm etc) to attack his physical form directly.
The whole thing where Jean walks out of the window, levitates there and gives off the Phoenix-shaped aura of fire is primarily to show the viewer that Jean is throwing around a lot of power and because, well it looks cool.